The present invention is directed to a bone plating system for fracture fixation, and in particular to a system including a bone plate having plate holes for both locking and non-locking screws.
The clinical success of plate and screw systems for internal fixation of fractures is well-documented. However, treatment of certain fractures, such as peri-articular fractures, which require a fixed angular relationship between the bone plate and screws, remains problematic. Fixed angle devices for treatment of these fractures are available and include the Dynamic Condylar Screw System commercially available from Synthes (USA) of Paoli, PA and a wide variety of blade plates. All of these devices require a high level of surgical skill, suitable bone quantity and quality, and a fracture pattern compatible with the device.
In cases in which these requirements are not satisfied, e.g. severely comminuted bone or missing bone segments, conventional bone plate and screw systems must be used. Although these conventional systems are particularly well-suited to promoting healing of the fracture by compressing the fracture ends together and drawing the bone into close apposition with other fragments and the bone plate, the angular relationships between the plate and screws are not fixed and can change postoperatively. This can lead to malalignment and poor clinical results.
The primary mechanism for the change in angular relationship is related to energy storage. As previously noted, threading a bone screw into bone compresses the bone against the plate. The compression results in high strain in the bone, and, consequently, energy storage. With the dynamic loading resulting from physiological conditions, loosening of the plate and screw and loss of the stored energy can result.
Securing the screws to the plate provides a fixed angle relationship between the plate and screw and reduces the incidence of loosening. One method of securing the screw to the plate involves the use of so-called xe2x80x9clocking screws.xe2x80x9d A locking screw has threading on an outer surface of its head that mates with corresponding threading on the surface of a plate hole to lock the screw to the plate. Bone plates having threaded holes for accommodating locking screws are known. For example, German Patent Application No. 43 43 117 discloses a bone plate with threaded holes for locking screws. As the relationship between the locking screws and the plate is fixed, locking screws provide a high resistance to shear or torsional forces. However, locking screws have a limited capability to compress bone fragments.
In summary, conventional bone screws, i.e. screws that are not secured to a plate so that a fixed angular relationship between the plate and screw is maintained (hereinafter xe2x80x9cnon-locking screwsxe2x80x9d) effectively compress bone fragments, but possess a low resistance to shear force that can lead to loosening of the screw. Locking screws have a high resistance to shear force that ensure stability at the bone screw/plate hole interface, but possess a limited ability to compress bone fragments. Thus, a bone plating system that combines non-locking screws with locking screws would be ideal for certain clinical situations.
U.S. Patent No. 5,601,553 discloses a locking plate and bone screw. The plate has a plurality of threaded plate holes for receiving locking screws. The plate also has non-threaded plate holes for receiving temporary screws that keep the plate in place while the locking screws are inserted. After the locking screws are inserted, the temporary screws are removed. Thus, the long term benefits of combining non-locking screws with locking screws are not obtained. U.S. Pat. No. 5,709,686 discloses a bone plate with partially threaded plate holes. The partially threaded holes allow either non-locking or locking screws to be used. Because the plate holes are only partially threaded, the locking screws used may not be able to maintain the fixed angular relationship between the screws and plate under physiological loads. Specifically, the locking screws within the plate are only partially captivated and thus only partially surrounded by threads. Under high stress and loading conditions, the locking plate hole may distort and allow the fixed angular relationship between the locking screw and plate to change. This can result in loss of fixation or loss of established intraoperative plate orientation. Additionally, because of the plate hole geometry, translation of the plate with the non-locking screws is limited to one direction only. This may be a disadvantage in reduction and manipulation of fragments.
Thus, there exists a need for an improved bone plating system that overcomes the deficiencies of the prior art.
The bone plating system for fixation of bone according to the present invention includes a bone plate having an upper surface, a bone-contacting surface, at least one first hole passing through the upper and bone-contacting surfaces and having a thread, and at least one second hole passing through the upper and bone-contacting surfaces. The bone plating system also includes a first screw having a shaft with a thread for engaging bone and a head with a thread configured and dimensioned to mate with the thread of the first hole, and a second screw having a shaft with a thread for engaging bone and a head. The first and second screws remain seated in their respective holes for substantially as long as the bone plate is implanted. Preferably, the bone plate includes a plurality of first and second holes, and a corresponding plurality of first and second screws are provided.
In order to facilitate insertion, the first and second screws can be a self-tapping screws. These screws can also be self-drilling screws. Additionally, the first and second screws can be cannulated for insertion of a guide wire to guide screw placement. The first plate hole can have a substantially conical shape with a double-lead thread.
In one embodiment, the bone plate has a trapezoidal shaped cross section in regions between the first and second plate holes for minimizing contact between bone and the bone-contacting surface. Additionally, at least one of the second plate holes is longitudinally elongated and has an edge inclined at an angle to the upper surface toward the bone-contacting surface for displacing the bone plate when engagged by the head of a second bone screw.
In an exemplary embodiment, the vone plate includes a head portion configured and dimensioned to conform to a metaphysis of a bone and a shaft portion configured and dimensioned to conform to a diaphysis of a bone. The head portion has only first plate holes and the shaft portion has both first and second plate holes. In one embodiment, the head portion has a curved surface, includes an anterior fork substantially parallel to an anterior side of the shaft portion, and includes a posterior fork extending out from a posterior side of the shaft portion. In another embodiment, the head portion flares outward from the shaft portion and is curved, tapered, and twisted. The head portion can also be provided with suture holes from suture anchoring of the bone plate.
The method for fracture fixation of bone according to the present invention comprises the steps of reducing the fracture to bring bone fragments in close apposition; compressing a bone plate against the bone with at least one first fastener to hold the fracture reduction; and securing at least one second fastener at a fixed angular relationship to the bone plate. The first fasteners are inserted before the second fasteners and both the first and second fasteners remain in bone for substantially as long as the bone plate is implanted.